Rear Windshield Circles

[AliLaPointe]

Did a couple of searches and couldnt find anything. I just washed my car, so I know that its not dirty, but the rear windshield has these circles all over it. Anyone know what these are?

[Zeuceone]

normal.

[Ruthless909]

[boltuprite]

you see, without knowing what we are looking at, how do you expect us to comment? Are they scratches or swirls embedded in the glass?
Are they big circles, or little circles.
Is it possible it's just tree sap drippings?

we just can't tell from your post.

[volvo9]

Are they kind of reflective?

I know what you are talking about, but I don't know what it is for, lots of cars have it though. Maybe an anti glare coating...?

[FG2uner]

my sunroof has circles on it.... they are from the suction cups on the assembley line machine that put the glass in. idk about yours if theyre the same or not.

[jthoske]

The "circles" you are seeing are as a result of the glass being tempered.

[Englishe30]

Originally Posted by FG2uner

my sunroof has circles on it.... they are from the suction cups on the assembley line machine that put the glass in. idk about yours if theyre the same or not.

I had the same thing...I took it to a detail shop and they got it off somehow...I couldn't get it off

[Adlh29]

Try rubbing alcohol on the sunroof and back windsheild. It worked for me.

[AliLaPointe]

I guess its only in certain lighting conditions. I just went and looked at the rear windshield and they were gone... really weird.

[Mabuz]

Try looking at your (and just about any newer car's) windows with some good quality polarized sunglasses on.

[kicker18]

Originally Posted by AliLaPointe

Did a couple of searches and couldnt find anything. I just washed my car, so I know that its not dirty, but the rear windshield has these circles all over it. Anyone know what these are?

I believe it is part of the rear defroster element, mine has the same condition.

[DuH-ReL]

Originally Posted by Mabuz

Try looking at your (and just about any newer car's) windows with some good quality polarized sunglasses on.

This is what I thought. The phenomena only occurs on some windows, and I think it's those which are produced with some chemical to reduce UV rays and/or heat from getting into the car. You don't often find it on all the windows of a car. I don't think it's due to it being tempered, because I believe all vehicles nowadays use tempered glass since it breaks into little pieces rather than shards that could cause greater injury.

[Ghetto2315]

Suction Cups

[c130turboprop]

window circles made by ufo's, just like crop circles, lol just kiddin, i have the same circles and ya, u can really only see them at certain sun/light positions and whatnot. i also have the ones on the sunroof too. i would say suction cups too from the assembly line, unless, just maybe, those damn aliens effin with us and doing thier dumb tricks on us, lol

[fern2021]

ok so it was either UFO's and aliens or suction cups. or also your window has been cursed and you need to replace it before the circles get bigger and eat your car (just my theory). usually the most simple explanation tends to be tho correct one.

[Mabuz]

Found this:

When viewed under particular lighting conditions, especially when viewed at a grazing angle, faint shadowy spots caused by the air quench process can often be seen in heat treated glass. These spots can be seen in transmission and in reflection, and at most viewing angles, other than directly facing the glass. They become very visible when polarizing sun glasses are worn. They are easily seen on a sunny day when the light comes from the blue sky or is reflected from clouds. They probably would not be visible on a grey, cloudy day. These spots are a normal function of properly tempered
glass and are mentioned in section 7.5 of ASTM C 1048 standard for heat treated glass. They are more visible when both lights of glass in an insulating glass (IG) unit are tempered. They are often seen in the sloping tempered back windows of cars, and in Asia they are very obvious in some small truck tempered front windshields.


Quench jets in a tempering furnace cannot cool the glass completely uniformly. As a result some areas are cooled faster than others resulting in differential shrinkage
which creates areas of different compressive stress. When glass is stressed in the quenching operation, with its corresponding very slight change in density, it becomes birefringant or polarizing, that is it partially blocks or passes polarized light. The greater the stress the greater will be the polarizing effect.

Vertically polarized light can easily pass through the vertically polarized sections of the glass to give bright spots but it is diminished when passing through other areas polarized at a different angle to give relatively darker spots. The spots are not an indication of absolute tempering level; they simply show areas of relatively more, or less, tempering stress. Given the nature of the quenching process it is physically impossible to quench a plate with absolute uniformity. The degree of tempering and the uniformity of tempering throughout the plate can be tested by other methods such as surface stress instruments or by examining the break pattern after fracture.

More reading: polarization of light: Definition and Much More from Answers.com

I also learned this is how 3D glasses work for 3D movies; one lense is polarized in one direction and the other lense is polarized 90 degrees from the first lense. This 90 degree difference only allows one eye to see what is meant for it, while blocking those images from the other eye, giving the effect of 3D.

So, auto window summary: The dots are there due to polarization to cut glare, but they can not entirely polarize a car's windows because if you were to drive with polarized sunglasses, you may not be able to see out of your window at all with them on due to the different anlges of polarization in the window and in your polarized sunglasses.

Class dismissed.